Multi-phase thyristor inverter

ABSTRACT

To simplify the reset circuitry of multi-phase thyristor inverters having two similar groups of thyristors, one group each controlling the connection of the phases of one polarity, a pair only of reset or quenching circuits are provided, one each output being connected to all the thyristors of a respective group to extinguish the thyristors of the group connected to provide output of the respective polarity during firing thereof. The circuit is preferably connected through an input choke having a plurality of windings in the form of current transformer windings, the additional windings being interconnected with the thyristor circuit to extend current flow through the chokes and provide for smooth commutation.

United States Patent 1 1 1111 3,733,539

Wirtz 14 1 May 15, 1973 1541 MULTl-PHASE THYRISTOR INVERTER 3,588,6676/1971 Duff et a1 ..321 45 [75] Inventor: Rainer Wirtz, Unterriexingen,Gcr- FOREIGN PATENTS 0R APPLICATIONS many v T 647,560 /1963 Belgium..321/45 C [73] Ass1gnee: Robert Bosch GmbH, Gerlmgenschmerhoehe,Germany Primary Examiner-William M. Shoop, Jr. 22 Filed: M 6, 1972Attorney- Robert D. Flynn et a1.

[21] Appl. No.: 232,093 57 ABSTRACT To simplify the reset circuitry ofmulti-phase thyristor Foreign App Priority Data inverters having twosimilar groups of thyristors, one Mar 24 1971 Germany P 21 14 098 7group each controlling the connection of the phases of one polarity, apair only of reset or quenching circuits are provided, one each outputbeing connected to all CCll. ..321/H the thyristors of a respectivegroup to atinguish the [58] Fieid 321/45 C thyristors of thegroupconnected to provide output of the respective polarity duringfiring thereof. The circuit is preferably connected through an inputchoke [56] References Cned having a plurality of windings in the form ofcurrent UNITED STATES PATENTS transformer windings, the additionalwindings being interconnected with the thyristor circuit to extend cur-3,399,336 8/1968 Koppelmann ..321/5 m fl h h the hoke and rovide forsmooth 3,110,817 11/1963 Frederick ...290 40 R Commutation 3,171,9663/1965 Bergslien et a1. ..290/40 R 7 Claims, 3 Drawing Figures /1 i1 5";n 0 l T f, D \;Q A;Q /4 E T "fi l 30 J? 40 i0 1 .92 5 I 12 re 0 U5 1! n44 .fii i1, f

F E 1 5/ if 7/ 9/ g a; E /e 1 MULTI-PHASE THYRISTOR INVERTER The presentinvention relates to a multi-phase thyristor inverter in which theinverter frequency is controlled, preferably a three-phase thyristorinverter to supply three-phase windings of asynchronous motors, tocontrol the operation of the motor from a d-c supply line.

Thyristor inverters have been previously proposed, see, for example, thebook Heumann-Stumpe: Thyristoren, 1969, pp. 183 to 188. Reference mayalso be had to General Electric SCR Manual, and particularly chapters onchoppers, inverters and cyclo converters; and turn-off characteristicsand methods.

Various types of turn-off circuits, which may briefly bereferred to asquenching circuits, have been proposed. Thyristor circuits in which thefiring cycle of the thyristors is controlled to provide a controlledoutput use different types of quenching or turn-off circuits dependingon the type of commutating or firing circuit used. In a simple case,sequential phase extinction is utilized, in which commutating condensersare arranged between the phases. Extinction of any thyristor in aninverter of this type is achieved by the firing of the thyristor of thesubsequent phase. This sequential phase on-off circuit does not,however, permit control of firing of the various phases with overlappedvoltage (or current) relationship. If overlapped control is desired,each one of the phases must have a separate and individual turn-offcircuit associated therewith. This is wasteful of equipment and theattendant maintenance and adjustment thereof.

Inverters which are utilized to control the speed of asynchronous motorsrequire not only change in the output frequency but in the outputvoltage as well. Output voltage can be varied by controlling supply tothe various phases not continuously but, rather, intermittently (in thisconnection, see particularly the above referred to General Electric SCRManual). To control the various thyristors of an inverter with a pulsefrequency which is high with respect to the output frequency of thethyristor, quenching or turn-off of the thyristor cannot be accomplishedby turning off the various phases by sequential turn-off, or by phaseturnoff circuits. Individual turn-off of the thyristors then requiresthat each individual thyristor of the inverter has its own individualquenching circuit. The quenching circuits require substantial additionalcomponents and circuitry in connection therewith.

It is an object of the present invention to provide an inverter circuitwhich has a turn-off circuit in connection therewith which permitscontrol of the various thyristors with overlap, so that the outputvoltage of the thyristor can be varied as desired, and in which thecomponents and circuitry to effect turn-off is substantially less thanif the thyristors are individually quenched.

Subject matter of the present invention: Briefly, a full-wave thyristorinverter is provided in which the thyristors are separated into twogroups, one each group providing a half wave of a predeterminedpolarity. Two turn-off circuits are provided, one for each groups of thethyristors providing the opposite half waves.

This circuit does not permit application of control pulses to thevarious phases in which the various phases have different mark-spaceratios (or different pulse widths). This, however, is not a disadvantagesince the phase outputs connected to asynchronous machinery are commonlysimilarly controlled. The circuit permits control of voltage andfrequency of the supply power to an asynchronous machinery withoutrequiring the number and complexity of individual turn-off circuits forindividual thyristors themselves.

The invention will be described by way of example with reference to theaccompanying drawings, wherein:

FIG. 1 is a circuit diagram of the first embodiment of the presentinvention;

FIG. 2 is a schematic illustration of a current transducer for use inthe circuit; and

FIG. 3 is a circuit diagram of another embodiment of the presentinvention.

Positive and negative supply terminals l1, 12 have a voltage U,,connected thereacross. The terminals 11, 12 can be connected to abattery, or, if the inverter is part ofa conversion apparatus, can beconnected to the output ofa rectifier circuit. Positive terminal 11 isconnected over a commutating choke, or reactor 13 to a positive bus 90.Negative terminal 12 is connected over commutating choke, or reactor 16to negative bus 91. A three-phase bridge inverter, including sixthyristors, which are preferably SCRs, 50, 53; 60, 63; 70, 73 isconnected between positive and negative buses 90, 91. The diagonal pointof the bridge provides the threephase output terminals R, S, T. A load,schematically shown as the armature windings 80, 81, 82 of anasynchronous machine is connected to terminals R, S, T. The circuit isuseful also with loads other than electrical machinery although it isparticularly applicable in connection therewith.

Six back-current diodes 51, 52; 61, 62; 71, 72 are connected into thebridge circuit, with reverse polarity. These diodes are not thenconnected to the positive and negative buses 90, 91 but are ratherconnected directly to the positive and negative terminals 11, 12.

A push-pull connected turn-off circuit including four SCR's 30, 31, 32,33 is connected between positive terminal 11 and negative bus 91. Thediagonal of the push-pull bridge commutating circuit includes acommutating condenser 34. This commutating bridge is connected toextinguish or turn off the first group of thyristors 53, 63, 73, whichare connected to the negative bus 91. A second push-pull turn off bridgecircuit having four thyristors 40, 41, 42, 43 is connected between thepositive bus 90 and the negative terminal 12. It further includes acommutating condenser 40. The bridge is connected to extinguish thesecond group of thyristors 50,60, 70, which are connected to thepositive bus 90.

The commutating reactor 13 has a secondary winding 14 wound on the corethereof. Winding 14 is connected at one end with negative terminal 12and on the second end to a clamping diode, or cut off diode 22, and thento input terminal 11. Similarly, the core of the second commutatingreactor 16 has a secondary winding 17 thereon which is connected at oneend to the positive terminal 11 and with the second end over a cut-offdiode 23 with the negative terminal 12.

Referring now to FIG. 3, the diagonal interconnection between thethyristors 50, 60, of one group, and the thyristors 53, 63, 73 of theother is indicated by lines 95, 96, 97. The currents flowing in lines95, 96, 97 are sensed in a transducer 54, 64, 74 and the sum of thetransducer-derived signals are connected to a terminal 92, to provide anoutput indication of the total power being delivered to the load. Thetransducers 54, 64, 74, which may be in the form of current transform.-ers or other suitable transducer elements are so connected that thealgebraic sum of the currents is being sensed, that is, both phase andpolarity being considered when supplying the output signal at terminal92.

FIG. 2 illustrates a single transducer which is particularly applicableto sense the currents in lines 95, 96, 97, this transducer being usefulin either embodiment of FIG. 1 or FIG. 3. Lines 95, 96, 97 areinductively coupled to a common iron core 93 which has an air gap inwhich a Hall detector 94 is located. The Hall detector 94, connected toterminal 92, then will measure the sum of the currents in the threelines 95, 96, 97, considering the direction of current as well asrelative phase. If current flows in one of the lines (for example line95) which is reversed with respect to the direction of the current inthe two other lines, then the effect of the current in line 95 will besubtracted from the effect of the currents in the other lines, so thatthe output will be truly representative of the algebraic sum of thecurrents. Rather than using a transducer with a Hall detector, othertransducers can be used.

The circuit of FIG. 3 is similar to that of the circuit of FIG. 1, andto the extent that it is identical will not be described again, and thesame reference numerals have been used. Lines 95, 96, 97 are connectedto the junctions of the thyristors of one group and the associatedback-current diodes (for example thyristor 50 and diode 51) so that thetotal current will be measured flowing in the respective phase, that is,not only the current due to the thyristor itself but also the currentflowing through the back diode. The circuit, in essence, is similarexcept that in the lines between the back diodes 51, 61, 71 and 52, 62,72, additional diodes 27, 24, respectively, are connected. Further, thecores of the commutating reactors 13, 18 have an additional tertiarywinding 15, 18, respectively, placed thereon. Winding is connected tonegative terminal 12 on one end, and at the other end to a third diodeand then to the junction between diode 27 and the back diodes 51, 61,71. Similarly, the tertiary winding 18 is connected to positive terminal11 on one end and at the other to a diode 26 which connects to thejunction of diode 24 and back diodes 52, 62, 72.

Operation: The basic operation of a three-phase controlled bridge isknown. The gates of the thyristors 50, 60, 70 and 53, 63, 73 aresuitably supplied with firing potentials to provide cyclical firingthereof, in dependence on control signals, derived from a firing controlsource (not shown) and known in the art. Upon firing ofa thyristor, forexample thyristor 50, phase terminal R is supplied with power for thepositive half wave. Upon firing of thyristor 53, the negative half waveis provided to terminal R. The amplitude of the supply voltage can bevaried such that the thyristor 50, or 53, respectively, fires, or isextinguished in predetermined timed relationship. The supply voltageincreases with increase of the ratio of pulse duration (mark") to pulseinterruption (space"). Thyristors 53, .63, 73, all connected to thenegative bus 91 and which, together, can provide a negative half wave,are all extinguished, together, by the first quenching bridge -34. Thequenching or turn-off circuit thyristor 53 is indicated in FIGS. 1 and 3in dashed lines.

Let it be assumed that, in advance of extinction, commutating condenser34 is charged as shown in FIG. 1, that is, its left terminal is chargednegatively, and its right terminal positively. If thyristors 30, 33 arefired (under control of an outside firing source controlling extinctionof the main thyristors), commutating condenser 34 will be connected overthyristor 30 and back diode 52 to the anode of the thyristor 53 which isto be extinguished; and further, over thyristor 33 connected to thecathode of thyristor 53 (see broken line circuit). Since commutatingcondenser 34 previously had been charged to the supply voltage U,,, thevoltage U,, is applied, in reverse direction, to the thyristor 53, thusblocking thyristor 53. Thyristors 30 and 33 then are effective to chargethe commutating condenser with reverse polarity, so that the rightterminal will now be negative. The next blocking is then done overthyristors 31 and 32.

In case one of the thyristors 63 or 73 were conductive together withthyristor 53, the additional thyristor, or thyristors are also blockedupon discharge of condenser 34. Thus, all thyristors of one group whichwere conductive are interrupted. If, during the same half wave,thyristor 53 should be additionally interrupted after further firing,the other thyristors 31, 32 of the first bridge should be fired in orderto have positive condenser potential available to be applied to thecathode of thyristor 53.

If the load is an asynchronous machine which is operating in brakingmode, it can be possible that all six thyristors 50, 53; 60, 63; 70, 73of the three-phase bridge are blocked simultaneously. Only the backdiodes 51, 52; 61, 62; 71, 72 will carry current derived from the motor,now acting as a generator. The braking energy is being supplied back tothe battery. Current measurement is obtained at terminal 92, as beforeindicated, for use in indicating or control functions.

The operation of the commutating reactors 13, 16 and of secondarywindings 14, 17 is known. During the above described quenching, thepolarity of the negative bus 91 is briefly reversed. The commutatingreactor 16 is used to prevent short-circuiting of the positive pulsearising at the cathode of thyristor 33 to the negative terminal 12. Uponquenching, the commutating reactor 16 will have a rise in currentoccurring therein. The magnetic energy stored in the reactor, or choke16 is delivered back over secondary winding 17 and cut-off diode 20 tothe source of direct potential. Similarly, protective reactor 13 isutilized to suppress negative voltage pulses on the positive bus 90. Themagnetic energy which is stored in reactor 13 is applied over secondarywinding 14 and cut-off diode 21 back to the source of direct current.

The quenching circuit for thyristor 53 is also shown in FIG. 3, whichincludes an additional tertiary winding in the choke. The diodes 24, 27are so poled that the quenching current for the thyristors will passtherethrough, that is, they are poled similarly to the back diodes towhich they are connected. The winding ratio between the various windingsof the reactors is preferably so selected that the turn ratio betweenwindings l6 and 17 is preferably between 2 l and 4 1; this ratio is,hereinafter, referred to by r. The tertiary winding 18 preferably hasthe same number of turns as that of the primary, that is, of winding 16.

During quenching, as above explained, the negative bus 21 will have apositive pulse (with respect to the negative terminal 12) appliedthereto. After reverse of the polarity on condenser 34, the secondarywinding 17 as well as the tertiary winding 18 will provide a negativevoltage pulse at the right terminal thereof. The negative voltage pulseof the secondary winding is carried along over the cut-off diode 23 tothe negative terminal 12. The negative pulse of the tertiary winding 18,however, is connected over diode 26 and back diode 52 to the phaseterminal R. This phase terminal R was supplied with negative currentuntil just before the extinction of thyristor 53. After this extinction,therefore, an additional negatively directed current is provided fromthe tertiary winding 18. This current, of course, decreasesexponentially. The embodiment of FIG. 1, also, provides a negativecurrent to the phase terminal R after extinction of the thyristor 53over back current diode. This current is due to the voltage provided bythe leakage induction of windings 80, 81, 82 of the rotating motor loadand by the voltage of commutating reactor 13, limited to the inductionvoltage (l/r) -U The second embodiment in accordance with FIG. 3provides a third voltage from the tertiary winding 15. If, for example,at first thyristors 53 and 70 are conductive, and thereafter thyristor53 is extinguished, motor current continues due to the inductivity ofthe motor and the induction voltage limited to (l/r) U to the secondcommutating reactor 13 in the following circuit:R-80-82-T--70-l3-24-52-R.

After change of polarity of commutating condenser 34 due to quenching,the dot terminal of tertiary winding 18 will likewise have a voltage of(1/r) U arising thereacross. This voltage is coupled over diode 26 intothe above given circuit and adds to the current due to the inductionvoltage of the first commutating reactor 13. Thus, the current withinthe above given circuit, that is, within the braking of free-wheelingcircuit will continue longer than in the embodiment in accordance withFIG. 1 which does not have a tertiary winding on the reactor.

If all thyristors are extinguished, for example during braking operationof the asynchronous machine represented by the load 80, 81, 82, andmotor current is fed back to the direct current terminals 11, 12, overthe back diode, the additional tertiary windings provide for an increasein driving voltage. The voltages of the tertiary windings will add tothe other voltages, particularly to the voltage of the asynchronousmachine now operating as a generator, so that the back currentattenuates more slowly.

The second embodiment (FIG. 3) permits setting of the trigger pulses tofire the thyristors at greater time intervals, that is, the variousthyristors are less often fired during any one half wave, so that thepower loss in the thyristors in the three phase bridge, as well as inthe extinguishing bridges is reduced.

In a preferred arrangement, the turns ratio of the windings should be 2l to 4 1 between the commutating windings l6 and secondary 13; if theratio is too small, for example 1 1, too much excess voltage will appearon the negative line 91 with respect to the negative terminal 12 afterthe thyristor 53 has extinguished. On the other hand, if the turns ratioshould be too great, then the magnetic energy within the reactor 16 willdecrease too slowly, and the repetition frequency of the extinction orblocking pulses cannot be selected to be sufficiently high.

The firing and extinction sequences of the other thyristors are similarto those of thyristor 53, above referred to, so that they need not bedescribed in detail. The voltage, as well as the frequency of the supplypower to the three-phase terminals R, S, T can be selected as desired,and the requirement with regard to components, and circuitry is muchless than inverters in which each of the thyristors have their own,individual quenching circuit. If each individual power thyristor wouldhave a separate quenching circuit, then six separate quenching bridgeswould be required. Instead of the two quenching bridges 30-34 and 40-44,other quenching circuits can be used as commonly known in controlledrectifier technology (see the above referred to SCR Handbook). Inaccordance with the invention, only one turn-off circuit is provided foreach group of thyristors providing a half wave of a predeterminedpolarity.

The thyristors of one group need not be just three; if six-phase a-c isto be generated, each group will have six thyristors; nevertheless, onlya single turn-off circuit is required for each group.

The tertiary windings l5, 18 decrease the current attenuation in thewindings 80, 81, 82 of the asynchronous machine during the pulseintervals of the thyristors. These thyristors thus need not be fired asoften, which improves the efficiency of the inverter as well as that ofthe motor.

The arrangement of a single transducer in accordance with FIG. 2 withinthe inverter having grouped extinction circuits is particularly simplesince a single measuring unit can measure the sums, that is, thealgebraic sums of all currents flowing in the motor. This is a sensedvalue which is completely sufficient for current regulation of theinverter, since, in any case, all thyristors of one group areextinguished simultaneously.

The firing circuits for the thyristors are known in the art, and maycomprise separate sources, or sources controlled by outside controllers,or placed in a feedback loop, which may, if desired, include a sensedquantity from the motor itself.

Various changes and modifications may be made within the inventiveconcept.

I claim:

l. Multi-phase thyristor inverter adapted to be connected to a source ofd-c potential and providing multiphase a-c output comprising two similargroups (50, 60, 53, 63, 73) of at least two thyristors each, thethyristors of one group being connected to the positive potential of thesource and the thyristors of the other group being connected to thenegative potential of the source, the groups of the thyristors beingconnected to provide multi-phase full-wave output (S, R, T);

a pair of switchable turn-off circuits (30-34; 40-44) having bi-polaroutputs, the output of one switchable turn-off circuit being connectedto all thyristors of a respective group and the output of the otherswitchable turn-off circuit being connected to all the thyristors of theother group to extinguish the thyristors in the respective group afterfiring thereof;

back-current diodes (51, 61, 71; 52, 62, 72) directly connected inopposite polarity in circuit with the respective thyristors and the d-csource, the output for each phase (S, R, T) being taken from theinterconnection (59, 96, 97) between the respective thyristor and theassociated back-current diode; and

current sensing means (54, 64, 74; 93, 94) coupled to saidinterconnection to sense the total current flowing in a respectivephase, the current sensing means having a common output (92) to providean output signal representative of total currentflow in the respectivephases.

2. Inverter according to claim 1, for use in a threephase network tosupply a motor, comprising at least three thyristors for each group, oneof each being connected to a phase of a respective phase winding of themotor;

the common output (92) from the current sensing means providing anoutput signal representative of total current flow in the motor.

3. Inverter according to claim 1, wherein the current sensing meanscomprises a core (93) having an air gap and conductors (95, 96, 97)carrying the currents of all the phases of the motor coupled to the coreso that the flux through the core will be representative of total motorcurrent; and

a Hall detector (94) in said gap, the Hall detector providing saidoutput signal.

4. Inverter according to claim 1, comprising a pair of commutatingreactors (13, l4; 16, 17), one each in series connection between thesource of d-c potential and the positive and negative supply to thegroups of thyristors, said reactors having secondary windings (14, 17),the secondary windings and the reactor being associated with a commoncore; and

a series of interconnections between the positive (11) and the negative(12) terminals of the source, each being connected to a respectivesecondary winding (14, 17) and a blocking diode (22, 23) connected tothe other terminal of the secondary winding and poled in blockingdirection with respect to the d-c source.

5. Inverter according to claim 1, including common connected diodes (24,27) poled similarly as said back current diodes interconnecting saidback current diodes and the positive and negative terminals of thesource.

6. Inverter according to claim 5, comprising a pair of commutatingreactors (13, 14; 16, 17), one each in series connection between thesource of d-c potential and the positive and negative supply of thegroups of thyristors, said reactors having secondary and tertiarywindings (15, 18), and respective tertiary diodes (25,

26) connected with the respective tertiary winding and to the junctionof the common connected diode (22,

27) and the back current diodes (51, 61, 71; 52, 62,

7. D-c to three-phase a-c inverter in combination with an 21-0 motorhaving at least three phase windings comprising two groups of at leastthree thyristors each (50, 60,

; 53, 63, 73), each group of thyristors supplying a half wave of thethree phase supply to the motor,

the thyristors of each group being connected with one terminal to arespective terminal of the d-c source,-the other terminals of thethyristor being connected to a phase winding (80, 81, 82) of the motor;and

two turn-off circuits, one for each group of thyristors, one eachcircuit being common to the thyristors of each group and comprisingswitchable quenching condenser means (30-34; 40-44) and diode means (51,61, 71; 52, 62, 72) one for each of the turn-off circuits and the groupsof the thyristors, the diode means being poled oppositely with respectto each other and to the thyristors to permit turn-off voltage from theturn-off circuits to be applied to all the thyristors of a group and toextinguish the thyristors of the group supplying opposite half waves tothe motor windings;

a commutating reactance in series with the connection from the d-csupply to the one terminal of the thyristors, said commutating reactancehaving an additional winding (15, 18) poled similarly to theseries-connected winding; and

diode means (24, 26; 25, 27) similarly poled and interconnecting theadditional winding with said diode means and the terminals of the d-csource, said diode means being poled to permit gradual decay of currentflow due to change of current flow through the commutating reactance andthus extend current flow through the windings of the molIOI'.

1. Multi-phase thyristor inverter adapted to be connected to a source ofd-c potential and providing multi-phase a-c output comprising twosimilar groups (50, 60, 70; 53, 63, 73) of at least two thyristors each,the thyristors of one group being connected to the positive potential ofthe source and the thyristors of the other group being connected to thenegative potential of the source, the groups of the thyristors beingconnected to provide multi-phase full-wave output (S, R, T); a pair ofswitchable turn-off circuits (30-34; 40-44) having bipolar outputs, theoutput of one switchable turn-off circuit being connected to allthyristors of a respective group and the output of the other switchableturn-off circuit being connected to all the thyristors of the othergroup to extinguish the thyristors in the respective group after firingthereof; back-current diodes (51, 61, 71; 52, 62, 72) directly connectedin opposite polarity in circuit with the respective thyristors and thed-c source, the output for each phase (S, R, T) being taken from theinterconnection (59, 96, 97) between the respective thyristor and theassociated back-current diode; and current sensing means (54, 64, 74;93, 94) coupled to said interconnection to sense the total currentflowing in a respective phase, the current sensing means having a commonoutput (92) to provide an output signal representative of total currentflow in the respective phases.
 2. Inverter according to claim 1, for usein a three-phase network to supply a motor, comprising at least threethyristors for each group, one of each being connected to a phase of arespective phase winding of the motor; the common output (92) from thecurrent sensing means providing an output signal representative of totalcurrent flow in the motor.
 3. Inverter according to claim 1, wherein thecurrent sensing means comprises a core (93) having an air gap andconductors (95, 96, 97) carrying the currents of all the phases of themotor coupled to the core so that the flux through the core will berepresentative of total motor current; and a Hall detector (94) in saidgap, the Hall detector providing said output signal.
 4. Inverteraccording to claim 1, comprising a pair of commutating reactors (13, 14;16, 17), one each in series connection between the source of d-cpotential and the positive and negative supply to the groups ofthyristors, said reactors having secondary windings (14, 17), thesecondary windings and the reactor being associated with a common core;and a series of interconnections between the positive (11) and thenegative (12) terminals of the source, each being connected to arespective secondary winding (14, 17) and a blocking diode (22, 23)connected to the other terminal of the secondary winding and poled inblocking direction with respect to the d-c source.
 5. Inverter accordingto claim 1, including common connected diodes (24, 27) poled similarlyas said back current diodes interconnecting said back current diodes andthe positive and negative terminals of the source.
 6. Inverter accordingto claim 5, comprising a pair of commutating reactors (13, 14; 16, 17),one each in series connection between the source of d-c potential andthe positive and negative supply of the groups of thyristors, saidreactors having secondary and tertiary windings (15, 18), and respectivetertiary diodes (25, 26) connected with the respective tertiary windingand to the junction of the common connected diode (22, 27) and the backcurrent diodes (51, 61, 71; 52, 62, 72).
 7. D-c to three-phase a-cinverter in combination with an a-c motor having at least three phasewindings comprising two groups of at least three thyristors each (50,60, 70; 53, 63, 73), each group of thyristors supplying a half wave ofthe three phase supply to the motor, the thyristors of each group beingconnected with one terminal to a respective terminal of the d-c source,the other terminals of the thyristor being connected to a phase winding(80, 81, 82) of the motor; and two turn-off circuits, one for each groupof thyristors, one each circuit being common to the thyristors of eachgroup and comprising switchable quenching condenser means (30-34; 40-44)and diode means (51, 61, 71; 52, 62, 72) one for each of the turn-offcircuits and the groups of the thyristors, the diode means being poledoppositely with respect to each other and to the thyristors to permitturn-off voltage from the turn-off circuits to be applied to all thethyristors of a group and to extinguish the thyristors of the groupsupplying opposite half waves to the motor windings; a commutatingreactance in series with the connection from the d-c supply to the oneterminal of the thyristors, said commutating reactance having anadditional winding (15, 18) poled similarly to the series-connectedwinding; and diode means (24, 26; 25, 27) similarly poled andinterconnecting the additional winding with said diode means and theterminals of the d-c source, said diode means being poled to permitgradual decay of current flow due to change of current flow through thecommutating reactance and thus extend current flow through the windingsof the motor.